Y-up to Z-up
OBJ files from rendering tools, scanners and model stores describe shape for the screen. A slicer needs to know how big the part is and which way is up. Converting to 3MF settles the first question, because the package states millimetres. The orientation option settles the second.
Many OBJ exporters put Y up, while slicers build along Z. Such a model lands on its back on the bed. The converter offers Rotate Y-up model to Z-up, which stands the part upright as it is written into the 3MF. If the part is still tilted, Edit → Rotate takes any angle and lay on bed drops its lowest point to Z = 0.
Size: decide the unit before saving
OBJ has no unit, and the viewer reads its numbers as millimetres. A scan in metres looks like a speck; a model from an inch-based program comes out 25.4 times too small. Measure one edge you know, then fix it with Edit → Size or Inches → mm. The 3MF writes the size you see in Info.
Converting OBJ to 3MF
- Drop the .obj file; its .mtl can come along in the same drop.
- Turn on Y-up to Z-up if the model lies on its side.
- Check the size, then the mesh check: scanned OBJ files often have holes that Repair closes.
- Press Download 3MF.
The OBJ and any files dropped with it are read in this browser tab and never uploaded.
Scans and sculpts: holes and triangle count
OBJ files from a scanner or a sculpting app bring two common problems to the print bed. The first is open edges, which slicers read as missing walls; Repair closes simple holes and reports what is still open. The second is a huge triangle count. Above roughly 1 to 2 million triangles the tab may run out of memory, and Reduce triangles trims the mesh to a percentage you set before the 3MF is written.
What the 3MF package records
A 3MF is a ZIP package, defined openly by the 3MF Consortium, with the model as XML at 3D/3dmodel.model. The unit is an attribute of that model. This converter writes millimetres there, which is the one thing an OBJ could never tell your slicer. The triangles are the same ones you see on screen after any edits.
If the 3MF arrives wrong in the slicer
Lying on its side means the Y-up option was off. In Info, the Up axis menu switches the view between Z up and Y up, so you can see the problem before you convert again with Rotate Y-up model to Z-up ticked in the Convert panel.
A size near 1 mm or near several metres means the unit guess failed; measure a known edge and use Edit → Size. A slicer warning about open or non-manifold edges sends you back to Repair mesh. If the OBJ itself will not open, check that the .obj file is complete; an .mtl that is missing only affects the look.
Weight and print cost before slicing
Info shows the volume and the weight for 17 materials, with a field for % solid to match your infill. The 3D print cost calculator adds price per kg and hours. A 3MF from this page keeps the millimetre size you set, so the slicer and the calculator see the same part. To slice in the tab, the mesh can go to STL to G-code.
Related conversions
If your slicer prefers a plain mesh, OBJ to STL does the same job without the package. For web viewers, OBJ to GLB. The reverse of this page is 3MF to OBJ, and the OBJ viewer shows the original file.
Questions and answers
Does the 3MF keep my OBJ textures?
Plan on geometry only. The viewer engine reads OBJ with MTL and textures, but our tests do not cover them yet, and the 3MF from this page carries the mesh.
Why is my OBJ lying on its side in the slicer?
It was exported Y-up. Turn on Rotate Y-up model to Z-up before downloading, or rotate it 90 degrees about X in Edit.
Can I convert an OBJ with several objects?
Yes. All objects go into the 3MF together, and Info shows how many separate parts the mesh has.
How large an OBJ can I convert?
There is no fixed cap. A 250,000-triangle model opened in about 1.5 seconds in our tests; files past 1 million triangles take longer and need a lot of memory.
Do I need to drop the .mtl file too?
Only for the look in the viewer. The 3MF is built from the geometry in the .obj, so a missing .mtl does not change the print file.
Is 3MF better than STL for an OBJ model?
For printing, 3MF has the advantage of a stated unit in millimetres. STL is the choice when a program does not read 3MF.